This invention relates to coastdown control for a vehicle engine and, more specifically, to coastdown control having controlled "drive-on" feel of the vehicle.
It is well known in automotive engine controls to regulate the released (typically referred to as closed) position of the throttle in the throttle bore of the engine such as by controlling the position of a movable throttle stop in order to achieve a desired engine operating condition. The most common function of such regulation is the closed loop control of engine idle speed such as by control of the throttle stop to regulate engine airflow.
The need for a controlled transition to the idle speed control mode when the vehicle operator releases the throttle has long been recognized. For example, to prevent the engine speed from undershooting the idle speed, or to prevent the increase in hydrocarbon emissions resulting from a deficiency of air during the coastdown period, it has been suggested that the released throttle position be established at some controlled transitional throttle angle.
One such known system for accomplishing this transitional control provides for controlling the throttle stop during vehicle coastdown to a predetermined coastdown schedule when the throttle is released and engages the throttle stop. The coastdown throttle angle control is continued until conditions for idle speed control are met at which time the idle speed control operating mode is enabled to control engine idle speed. The angle of the throttle established during the coastdown period is a function of parameters that typically include engine temperature, electrical loading, etc.
During vehicle coastdown, the vehicle ride quality as perceived by the vehicle operator is a function of the "drive-on" feel of the vehicle. Excessive engine braking (the engine being driven by the wheels through the transmission during closed throttle coastdown and therefore retarding vehicle motion) or sail on (excessive engine output resulting in the wheels being driven by the engine during coastdown). Each of these drive-on conditions may result in a perceived poor vehicle ride quality.